When you're in Sofia, you must, must, MUST see Alexander Nevsky Cathedral. It's truly amazing! Try to get a chance to go inside and check it out. The sight will speak for itself... I also reccomend you to see the National Theater 'Ivan Vazov', the National Palace of Culture, the Rotunda (old Roman building from more than a millennium ago) and walk around Vitosha Boulevard and Graf Ignatiev Street.